This invention relates generally to the rendering of latent electrostatic images visible on a charge retentive surface using multiple colors of dry toner or developer supplied by a plurality of developer housings and more particularly to the reduction of interaction between an image rendered visible by developer material supplied by one of the developer housings and developer materials contained in another developer housing.
The tri-level highlight color xerographic process is one method of making single pass, two color images. The basic concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. In this process, the latent image is created by first charging the photoreceptor (p/r) to some initial charge level (V.sub.o), and then exposing the p/r to three discrete voltage levels using a Raster Output Scanner (ROS). The two voltages that represent the document information (both colors) are commonly referred to as the Charged Area Development potential (V.sub.CAD) and the Discharged Area Development potential (V.sub.DAD). The third voltage represents the white or background potential (V.sub.WHITE), and corresponds to the background areas or those areas of the document that are to be white. V.sub.CAD is generated when the ROS output is minimum (off), and is roughly equal to V.sub.o. V.sub.DAD, on the other hand, is generated when the ROS output is maximum (on full), as is typically equal to the residual potential of the p/r (&lt;100v). V.sub.WHITE is generated when ROS output is approximately at half power, and is typically equal to V.sub.0 /2.
Once the tri-level latent image is formed, it is then developed by passing it sequentially through or past two independent developer housings, each containing one of the two required developers. In theory, either of these housings can contain either color developer, and either color developer (specifically, the toner) can be either positive or negative in charge, as long as the two developers are opposite in polarity. For the purpose of this disclosure, it will be assumed that a black developer with positive toner resides in the first housing, and a color developer with negative toner resides in the second housing. Preferably the two developer housings contain conductive magnetic brush developer.
As the latent image passes in close proximity to the first housing, the positive black toner is attracted to and finally deposited in the more negative areas of the p/r, called V.sub.CAD, and development continues until the V.sub.CAD surface potential roughly equals that of the first developer housing bias (V.sub.CAD bias). This bias, which is typically .apprxeq.100v more negative then V.sub.WHITE, creates a cleaning field between this housing and both V.sub.WHITE and V.sub.DAD, thus suppressing development of black toner in these areas. When the latent image is passed through the second housing, the negative color toner is deposited in the less negative areas of the p/r, called V.sub.DAD, until the V.sub.DAD surface potential roughly equals that of the second housing bias (V.sub.DAD bias). This bias is typically .apprxeq.100v less negative then V.sub.WHITE, and creates a cleaning field between this housing and both V.sub.WHITE and the residual V.sub.CAD which suppresses development of the negative color toner in these areas.
After development of the tri-level image is complete, one additional step must be implemented prior to transfer. Because the developed image contains toner of both signs (i.e. positive and negative), it must be exposed to a pre-transfer corona (either positive or negative) to make the toners common in sign. Once this is done, the image can then be transferred to paper using conventional electrostatic transfer.
When the tri-level latent image is passed through the first developer area, the V.sub.CAD portion of the latent image is developed with black toner. As development takes place, V.sub.CAD is reduced in amplitude by a process called neutralization, which is the pairing of negative charges on the p/r with positive charges on the toner particles. In theory, total neutralization (100% ) of V.sub.CAD is achieved when enough positive toner is deposited on the p/r to make V.sub.CAD =V.sub.CAD bias. In practice, however, total neutralization is rarely achieved, and the Post Development (PD) V.sub.CAD is typically 30 volts more negative then V.sub.CAD bias.
A typical residual tri-level image after development by the first housing which comprises the combination of the residual V.sub.CAD after development, defined as (V.sub.CAD (PD)+(V.sub.CAD bias-V.sub.WHITE)), is on the order of 130 volts. When the latent image is passed through the second housing (which contains the DAD color developer in this case), the presence of the residual V.sub.CAD causes high cleaning fields between this residual and the V.sub.DAD bias. These cleaning fields, coupled with the weakened magnetics (almost field free) employed in the second housing to minimize the disturbance of the developed CAD image, have the following undesired effects:
1. Because the DAD carrier beads are positive in charge, they are attracted to the more negative regions of the p/r surface. The most negative areas on the p/r prior to entering the second housing are the CAD(PD) areas, and as a result are the areas most likely to suffer deposition of DAD developer beads. The presence of beads in these regions results in large deletions in the CAD image when the image is transferred to paper.
2. Because of the weak magnetics used in the second housing, the effective conductivity of the DAD developer is lower than it would be if it were in the first housing (which uses full strength (i.e. conventional magnetic brush development) magnetics)). As a result, the DAD developer is more likely to respond to fringe fields, such as the ones that exist between V.sub.CAD (PD) and V.sub.WHITE, causing color toner to be deposited around the outside of the CAD image areas. This type of deposition has been observed in the past on actual tri-level prints.
3. Any wrong sign toner (positive) contained in the DAD developer will be attracted to, and possibly deposited in, the residual V.sub.CAD areas. While this may not be detrimental if the DAD developer is a color (i.e. red in the black is hard to see), it is very harmful if the DAD developer is black.
Various techniques have been employed in the prior art to minimize the disturbance of the first developed image by developer materials in the another housing by which the developed image must pass. By in large such techniques have dealt with the modification of the development apparatus of the second developer system. For example:
There is disclosed in U.S. Pat. No. 4,308,821 granted on Jan. 5, 1982 to Matsumoto, et al, an electrophotographic development method and apparatus using two magnetic brushes for developing two-color images which do not disturb or destroy a first developed image during a second development process. This is because a second magnetic brush contacts the surface of a latent electrostatic image bearing member more lightly then a first magnetic brush and the toner scraping force of the second magnetic brush is reduced in comparison with that of the first magnetic brush by setting the magnetic flux density on a second non-magnetic sleeve with an internally disposed magnet smaller than the magnetic flux density on a first magnetic sleeve, or by adjusting the distance between the second non-magnetic sleeve and the surface of the latent electrostatic image bearing members. Further, by employing toners with different quantity of electric charge, high quality two-color images are obtained.
U.S. Pat. No. 3,457,900 discloses the use of a single magnetic brush for feeding developer into a cavity formed by the brush and an electrostatic image bearing surface faster than it is discharged thereby creating a roll-back of developer which is effective in toning an image. The magnetic brush is adapted to feed faster that it discharges by placement of strong magnets in a feed portion of the brush and weak magnets in a discharge portion of the brush.
U.S. Pat. No. 3,900,001 discloses an electrostatographic developing apparatus utilized in connection with the development of conventional xerographic images. It is utilized for applying developer material to a developer receiving surface in conformity with an electrostatic charge pattern wherein the developer is transported from the developer supply to a development zone while in a magnetic brush configuration and thereafter, transported through the development zone in a magnetically unconstrained blanket contact with the developer receiving surface.
Disclosed in a patent application (Attorney's Docket No. D/86213) assigned to the same assignee as the instant invention, is a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic rolls associated therewith. The magnetic rolls disposed in a second developer housing are constructed such that the radial component of the magnetic force field produces a magnetically free development zone intermediate a charge retentive surface and the magnetic rolls. The developer is moved through the zone magnetically unconstrained and, therefore, subjects the image developed by the first developer housing to minimal disturbance. Also the developer is transported from one magnetic roll to the next. This apparatus provides an efficient means for developing the complimentary half of a tri-level latent image while at the same time allowing the already developed first half to pass through the second housing with minimum image disturbance.
As disclosed is U.S. Pat. No. 4,486,089 granted on Dec. 4, 1984 to Itaya, et. al. a magnetic brush developing apparatus for a xerographic copying machine or electrostatic recording machine has a sleeve in which a plurality of magnetic pieces are arranged in alternating polarity. Each piece has a shape which produces two magnetic peaks. The sleeve and the magnets are rotated in opposite directions. As a result of the above, it is alleged that a soft developer body is obtained, and density unevenness or stripping of the image is avoided.
While my invention contemplates the use of a modified second developer apparatus it also contemplates the use of a scorotron discharge device for neutralizing of the first residual latent electrostatic image to further minimize the interaction between developer materials contained in a second developer housing and the image already developed by the first developer housing.
It is known in the prior art to expose the charge retentive surface containing a developed image corona discharge. As illustrated in U.S. Pat. No. 4,660,961 granted to Kuramoto et. al. on Apr. 28, 1987, a charging assembly is employed between two developer housings for providing additional uniform positive charge to the photosensitive surface used therein.
U.S. Pat. No. 4,562,130 granted to Tateki on Dec. 31, 1985 discloses the use of a scorotron device which is utilized for stabilizing an unstable intermediate potential on a charge retentive surface for the purpose of enabling the setting of developer bias voltages. The unstable potential area is raised to the grid voltage of the scorotron by exposure of the charge retentive surface to the scorotron discharges. The use of such a scorotron device is also disclosed is U.S. Pat. Nos. 4,525,447 granted to Tanaka on June 25, 1985 and 4,539,2181 granted to Tanaka on Sept. 3, 1985.
U.S. Pat. No. 4,308,821 granted to Matsumoto et al on Jan. 5, 1982 disclose the differential charging of developer material in order to obviate materials interaction due to the stronger attractive forces of the one material and the charge retentive surface.